The present invention relates to a polishing head assembly for use in an apparatus for the chemical mechanical planarization (CMP) of surfaces such as semiconductor wafers.
CMP is used to planarize raw semiconductor wafers and each layer of material added to the semiconductor wafer thereafter. Conventional CMP systems often use a rotating wafer holder that brings the semiconductor wafer into contact with a polishing pad. The polishing pad moves in the plane of the semiconductor wafer surface to be polished. A polishing fluid, such as a chemical polishing agent or slurry containing microabrasives, is applied to the polishing pad. The wafer holder then presses the semiconductor wafer against the rotating polishing pad, and polishing commences.
During polishing, a semiconductor wafer is held by a polishing head apparatus. The semiconductor wafer is shaped like a disk, having a surface to be polished, a backside opposite that side, and outer perimeter. The backside of the semiconductor wafer is held against a carrier head through vacuum ports, and the outer perimeter of the semiconductor wafer is held with a retainer ring.
During polishing, a spindle rotates the semiconductor wafer as the spindle applies a downward force on the carrier head, pushing the semiconductor wafer against the polishing pad. The spindle has a diameter that is smaller than the diameter of the carrier head. Thus, the force on the semiconductor wafer tends to be greater in the center than around its perimeter. This can cause the perimeter of the semiconductor wafer to flare up, away from the polishing pad while the center region is being pressed more forcefully against the pad. This uneven force distribution can also cause uneven polishing of the semiconductor wafer.
Methods and apparatuses are needed to compensate for the uneven force distribution in existing CMP systems.
The present invention solves at least some of the problems left unsolved by existing carrier heads, retainer rings, and CMP apparatuses.
In one aspect of the invention, a polishing head assembly for use in a chemical mechanical planarization apparatus is provided. The polishing head assembly comprises (a) a carrier head shaped substantially like a disk having a circumference, a top surface, a bottom surface, and an outer wall, the outer wall having a groove therein, the groove extending into the carrier head from the bottom surface of the carrier head, and the groove running the entire circumference of the carrier head; and (b) a retainer ring having an interior wall and an exterior wall, the interior wall of the retainer ring being in contact with the outer wall of the carrier head, the interior wall having a slot therein, the slot defining a lower portion of the interior wall as a flexible leg, the flexible leg having a receiving end that is adapted to secure an object having a surface to be polished, the slot having a first terminal end adjacent to the groove in the carrier head and a second terminal end, opposite the first terminal end, in the body of the retainer ring.
In another aspect of the invention, a carrier head for use in a polishing head assembly is provided. The carrier head is shaped substantially like a disk having a circumference, a top surface, a bottom surface, and an outer wall, the outer wall having a groove therein, the groove extending into the carrier head from the bottom surface of the carrier head, and the groove running the entire circumference of the carrier head.
In still another aspect of the invention, a retainer ring for use in a polishing head assembly is provided. The retainer ring has an exterior wall and interior wall, the interior wall of the retainer ring having a slot therein, the slot defining an upper portion of the interior wall adapted to grip a carrier head and a lower portion of the interior wall as a flexible leg, the flexible leg having a receiving end that is adapted to secure an object having a surface to be polished, the slot having a first terminal end adjacent to the groove in the carrier head and a second terminal end, opposite the first terminal end, in the body of the retainer ring.
In yet another aspect of the invention, a method of compensating for uneven force distribution in a chemical mechanical planarization apparatus is provided. The method comprises (a) providing a chemical mechanical planarization apparatus having a head retainer, a carrier head in the retainer, and a retainer ring around the carrier head wherein a downward force is applied more forcefully in a central region of carrier head than in a perimeter region of the carrier head; (b) making a groove in the carrier head around the perimeter of the carrier head; (c) making a slot in the retainer ring that divides the retainer ring into an upper portion in contact with the carrier head and a lower portion that is a flexible leg having a receiving end that is adapted to receive an outer perimeter of a semiconductor wafer, the slot having a terminal end adjacent to the groove; and (d) securing a wafer against the carrier head such that the receiving end of the flexible leg grips semiconductor wafer and can move the outer perimeter of the semiconductor wafer to compensate for the uneven force distribution of the chemical mechanical planarization apparatus.